Hinged doors are commonly barred closed for security reasons. The presence of the bar extending across the door opening prevents the door from swinging open under forces applied from the opposite side of the door. The usual arrangement includes receptacles mounted on the door jamb, or adjacent wall structure, at or slightly above the height of the door handle. These receptacles are engaged by a solid or telescoping bar. Solid bars have to be lifted out of hook-shaped receptacles, and the telescoping bars may have internal compression springs biasing the bar sections to increased overall length to keep them in engagement with the receptacles. A cross-pin usually engages aligned holes in the bar sections to reinforce the effect of the spring. Release is obtained by pulling the pin, and then pulling the bar sections to decreased length against the spring action, so that the bar can be disengaged from the receptacles.
In case of fire or other emergency, it becomes necessary to release the door immediately, often under panic conditions. People cannot then be counted upon to act with calm logic, and complications involved in the forcible disengagement of a security bar may be too much to handle.